1. Field of the Invention
This invention relates to a copper-containing oxide material useful as a superconductor or as a parent phase convertible to a superconductor by carrier doping.
2. Description of the Prior Art
One known oxide having an infinite layer crystal structure is of a (Ca, Sr)CuO.sub.2 system as shown in FIG. 2 which is produced at a super-high pressure of 60,000 atm and a high temperature of 1,000.degree. C. or more (M. Takano et al, Physica C 159, 375(1989)). The illustrated structure contains one CuO.sub.2 plane and one cation plane in the unit cell. The cation cite is occupied by a plurality of atoms in the form of a solid solution. Under ambient pressure, an oxide with such an infinite layer structure must be prepared at a temperture of 950.degree.-1,150.degree. C. and, therefore, only available is an oxide of the formula Ca.sub.0.84 Sr.sub.0.16 CuO.sub.2 (T. Siegrist et al, Nature 334, 231 (1988)). This oxide, which contains a CuO.sub.2 plane commonly included in ceratin superconductors, was expected to be a mother phase convertible to a superconductive material upon carrier doping. However, superconductor derived from the oxide has not yet been produced because of difficulties in element substitution.
There are some reports disclosing that superconducting oxides having an infinite layer structure may be produced at a super-high pressure of 20,000-60,000 atm. Namely, the sintering of a composition of Nd.sub.x Sr.sub.l-x CuO.sub.2 (0.14.ltoreq..times..ltoreq.0.16) at 25,000 atm and below 1,000.degree. C. gives a superconductor having Tc of 40K (M. G. Smith et al, Nature 351, 549 (1991)). An infinite layer superconductor of the Ba-Sr-Cu-O system is produced by a similar high pressure technique (M. Takano et al, Physica C 176, 441 (1991)). In these methods, a super-high pressure is used to extend the solid solution limit of the crystallographic phase and, thereby, to control the atoms entering the cation site located between the two CuO.sub.2 planes.
It is reported that cation orderings are important to obtain superconductivity of a compound such as (La, Sr).sub.2 CaCu.sub.2 O.sub.6 (T. Sakurai et al, Physica C 174, 187 (1991)).